During the past decade, soft lithography has developed to a versatile technique for fabricating micro- and nanostructured surfaces[1;2]. Among several techniques known collectively as soft lithography, micro contact printing (μCP) has become the most commonly used method[1]. The technique initially developed for the transfer of molecules was later applied for the transfer of metals[3] in the form of nanotransfer printing (nTP)[4-6] and soft-contact lamination (ScL)[7].
Nanotransfer printing and soft-contact lamination are both schematically depicted in FIG. 1. In case of nTP (FIG. 1a), a thin layer of metal is evaporated onto a patterned elastomeric stamp, which has been fabricated by drop casting of polydimethylsiloxane (PDMS) onto a patterned Si wafer. The evaporated metal layer is brought into conformal contact with an organic layer on the substrate. As a result of the chemical bond formation at the metal/organic interface, the metal layer is transferred from the PDMS stamp onto the organic layer. The process takes place under ambient conditions without application of any additional pressure. Using this process Au top electrodes in Au/alkanedithiol/GaAs hetero junctions[5] and Au/mercaptosilane/Si hetero junctions[8] were printed. In a slightly modified process, gold is patterned on silicon wafers and subsequently transferred to selected polymers at high pressures (100-400 psi) and temperatures ranging from 100 to 140° C.[9].
In case of ScL the metal-organic adhesion is based on van-der-Waals interactions and weaker than the metal-PDMS interaction. Thus, in this process the metal is not transferred from the PDMS onto the organic layer, but the PDMS remains on the Au layer and is part of the PDMS/metal/organic/substrate junction (FIG. 1b). The process takes place in ambient conditions without the application of any additional pressure[10].
Besides nTP and ScL, a printing technique called polymer assisted lift off (PALO) printing has been developed[11] recently (see FIG. 2). As for its predecessor, the lift-off/float-on technique[12], a metallic top electrode is deposited and patterned on a solid substrate. The electrodes are then covered by a thin layer of PMMA (Polymethyl methacrylate) and released from the substrate by shortly immersing them into KOH and acetic acid. The resulting PMMA foil with electrode structures underneath can finally be placed onto the final substrate, i.e. a molecular layer. However, this transfer of the electrode structures has to be done in a water bath, which does not allow for precise alignment between substrate and top electrode.
In addition to these printing techniques, a gentle process that is scalable down to critical dimensions of <50 nm has been developed by some of the present inventors[13] (European Patent Application No. 06 006 899.6, filed on Mar. 31, 2006). Patterned metal structures are defined on a solid substrate and transferred onto a polymer pad (FIG. 3a). Subsequently, these metal patterns can be transferred from the polymer pad onto other surfaces such as SiO2 (FIG. 3b).
For molecular electronics, the PALO technique has the advantage to be very soft and gentle, so that the molecular layer is not damaged during the deposition of the top electrode. However, PALO does not allow for a precise alignment of the pattern to be printed with the substrate and any structures thereon. For example if the pattern to be printed is a set of top electrodes, and these are to be printed onto a substrate on which there are already located bottom electrodes, some of the aforementioned techniques will prove difficult to achieve a precise alignment. Furthermore, the polymer assisted lift of (PALO) has proved to be rather difficult to reproduce in that the polymethyl methylacrylate foil is prone to breakage of foils and/or patterns. Furthermore the PALO technique relies on shortly immersing the substrate in KOH and acetic acid which may cause unwanted side reactions on the pattern to be printed. For example if the pattern to be printed is a metal pattern, the presence of KOH and acetic acid may cause oxidation reactions taking place on the metal and thereby contaminate the electrodes.